Recent years have seen an increasing number of cases where an electric power user introduces a charging system capable of power peak shaving. In this type of charging system, a rechargeable battery is charged during a time zone in which power usage is low, and electricity thus charged is discharged from the rechargeable battery during a time zone in which the power usage reaches its peak so that peak power of the power usage is reduced. Furthermore, there have also been an increasing number of cases of introducing a charging system in which a rechargeable battery is charged with less costly midnight electricity, and the electricity is discharged in the daytime so that a daytime electricity cost is reduced. The introduction of these types of charging systems allows a total electricity cost to be reduced.
As a rechargeable battery that is incorporated into such a charging system, typically, a secondary battery such as a lithium-ion battery, a nickel metal hydride battery, a lead battery, or the like is used. A lithium-ion battery, in particular, is lightweight and high in capacity and in output and thus has been receiving attention as a rechargeable battery for the above-described types of charging systems.